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• Soil Stockpile SAE A soil stockpile will be located as shown on the Drainage and Sediment Control Plan (Map 2.05-M4). The stockpile will encompass approximately 2.24 acres. Once soil is placed in the stockpile, it will be seeded with the previously approved interim seed mixture and mulching at an application rate of 1 ion per acre. Therefore, the soil stockpile will be in a disturbed condition For a relatively short time. Due to the small area affected by the soil stockpile, conventional drainage and sediment control measures are not necessary. Oxbow proposes alternative sediment control measures to contain runoff and sediment from the soil stockpile area. A clean water diversion channel, labeled Ditch D7, will be constructed up-slope of [he stockpile to divert runoff from the undisturbed watershed above the pile. A conservatively designed 3.0- foo[berm will be constructed down-slope of the stockpile. The berm will contain [he entire runoff volume from a 10-year, 24-hour (2.0 in.) storm event. Runoff volumes were estimated using [he SEDCAD4 computer model. Water stored behind the berm will be allowed to slowly drain through straw bales positioned a[ the north (down-gradient) end of the berm. The containment berm and diversion channel are shown on Maps 2.05-MI & M4. The Ditch D7 design is provided in this Appendix. The following calculations demonstrate that a 3.0-foot containment berm constructed down-gradient of the stockpile will contain the runoff from the design storm. Hvdroloev Inputs The curve number (CN) assigned [o the soil stockpile was indicative of a one season reclaimed condition using approved interim seed mix. . Reclaimed Soil Type: C Ground Cover: Herbaceous Cover Type: Poor < 50% CN: 87 All other hydrology inputs are shown on the attached SEDCAD4 computer printouts. Hvdroloay Results Runoff volume: 0.17 ac. ft. (7,405 cu. ft) Berm Design A 3.0-foot high berm will be constructed on [he down-gradient side of the stockpile along the contour for its full length. The length of the stockpile along the down-gradient contour is estimated to be 540 ft. The slope of [he land is estimated [o be 25%, on average. The capacity of the berm was calculated as follows: Cross-sectional area (containment area): Berm Heigh[ (2H:1 V) 3.0 ft. (use H=2.7 ft. for capacity calcula[ion/0.3 ft, freeboard); Using A=0.5(LxH) Length (L), using 25% average slope, 25%=14 degrees, L = 2.7 ft.l1'an 14 = 10.8 ft Cross-sectional Area (A) _ (2.7 ft X 10.8 ft)(0.5) = 14.6 sq.ft. Capacity of Berm: (AxLength of berm) 14.6 sq. ft. X 540 ft = 7,884 cu. ft. = 0.18 ac. ft. Since 0.17 ac. ft. is required and 0.3 ft. of freeboard is provided, the containment capacity is adequate. Based on [he small size of [he area, establishment of interim vegetation, and OMI's ability to effectively control runoff and sediment with alternate sediment controls, an SAE is requested for [he soil stockpile area.